Liquid-crystalline behaviour of molecules with tetrahedral symmetry

Abstract

It is shown that molecules with tetrahedral symmetry can give smectic liquid-crystalline phases, both with liquid and with ordered layers. In contrast to the usual behaviour the layering is rather strong, leading to three orders in the X-ray diffraction pattern. A tentative model is given in which the long mesogenic substituents are bent at the spacers with which they are attached to the central carbon atom, such that in spite of the tetrahedral angles a rod-like shape is obtained.     

Synthesis and self‐assembly of helical polypeptide‐random coil amphiphilic diblock copolymer

Three amphiphilic rod‐coil diblock copolymers, poly(2‐ethyl‐2‐oxazoline‐b‐γ‐benzyl‐L ‐glutamate) (PEOz‐b‐PBLG), incorporating the same‐length PEOz block length and various lengths of their PBLG blocks, were synthesized through a combining of living cationic and N‐carboxyanhydride (NCA) ring‐opening polymerizations. In the bulk, these block copolymers display thermotropic liquid crystalline behavior. The self‐assembled aggregates that formed from these diblock copolymers in aqueous solution exhibited morphologies that differed from those obtained in α‐helicogenic solvents, that is, solvents in which the PBLG blocks adopt rigid α‐helix conformations. In aqueous solution, the block copolymers self‐assembled into spherical micelles and vesicular aggregates because of their amphiphilic structures. In helicogenic solvents (in this case, toluene and benzyl alcohol), the PEOz‐b‐PBLG copolymers exhibited rod‐coil chain properties, which result in a diverse array of aggregate morphologies (spheres, vesicles, ribbons, and tube nanostructures) and thermoreversible gelation behavior. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3108–3119, 2008     

Liquid-crystalline behaviour of molecules with tetrahedral symmetry

It is shown that molecules with tetrahedral symmetry can give smectic liquid-crystalline phases, both with liquid and with ordered layers. In contrast to the usual behaviour the layering is rather strong, leading to three orders in the X-ray diffraction pattern. A tentative model is given in which the long mesogenic substituents are bent at the spacers with which they are attached to the central carbon atom, such that in spite of the tetrahedral angles a rod-like shape is obtained.     

Liquid-crystalline behaviour of benzobisthiazole derivatives as a rigid rod polymer model

Novel materials with a new type of rigid core, namely benzobisthiazole, have been synthesized. The exhibition of liquid-crystallinity is found to be dependent upon the linkage between the rigid core and the alkyloxy phenyl terminal moieties. An interesting feature is the occurrence of tilted smectic phases (smectic C and smectic F) even though there is no significant central dipole moment transverse to the molecule.     

Networks of rigid molecules

Networks of rigid molecules do not fit into the paradigm of classical theories of rubber elasticity. Some experimental properties of rigid polyamide networks are summarised, and the basis for a theoretical understanding of such systems is discussed and elaborated in terms of the properties such as the existence of several conformational states and twist of such network elements.     

Ordered nanostructures from self‐assembly of H‐shaped coil–rod–coil molecules

A new class of π‐conjugated, skewed H‐shaped oligomers, consisting of biphenyl, phenylene vinylene, and phenylene ethynylene units as the rigid segment, were synthesized via Sonogashira coupling and Wittig reactions. The coil segments of these molecules were composed of poly(ethylene oxide) (PEO) or PEO with lateral methyl groups between the rod and coil segment, respectively. The experimental results revealed that the lateral methyl groups attached to the surface of the rod and coil segments dramatically influenced the self‐assembling behavior of the molecules in the crystalline phase. H‐shaped rod–coil molecules containing a lateral methyl group at the surface of the rod and PEO coil segments self‐assemble into a two‐dimensional columnar or a three‐dimensional body‐centered tetragonal nanostructures in the crystalline phase, whereas molecules lacking a lateral methyl group based on the PEO coil chain self‐organize into lamellar or hexagonal perforated lamellar nanostructures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 85–92     

Liquid-crystalline octopus dendrimers: block molecules with unusual mesophase morphologies

The synthesis and the mesomorphic properties of several new main-chain liquid-crystalline dendrimers, thereafter designated as octopus dendrimers in accordance with their eight sidearms, are reported. In these dendritic systems, the arborescence is ensured by anisotropic segments, acting as branching cells with a double multiplicity, which are incorporated at every node of the dendritic architecture. In such a way, these compounds radically differ from the classical end-functionalized liquid-crystalline dendrimers, the most commonly reported systems. Following our previous report on purely homolithic systems, that is, the building blocks constituting the dendritic matrix are all identical, several heterolithic systems made of different anisotropic blocks have been prepared. The dendritic branches and corresponding dendrimers were synthesized using a modular construction. Polarized optical microscopy and X-ray diffraction studies showed that all of these new octopus dendrimers exhibit either smectic-like or columnar phases with novel morphologies, the nature of the mesophases depending on the number of terminal chains attached to the peripheral groups. The mesomorphism of these heterolithic dendrimers is discussed in terms of their intrinsic architecture and compared to the analogous homolithic octopus systems. Models for the molecular organizations within both the smectic and the columnar phases are proposed on the basis of small Bragg angle X-ray diffraction studies and are supported by molecular modelizations. Moreover, this study showed that the mesophase stability is very sensitive to the nature and to the mutual arrangement (the spatial location) of the mesogenic segments within the dendritic matrix, illustrating the intimate relationships existing between the mesomorphic properties and the molecular architecture of these dendrimers.     

Surface-functionalized CdSe nanorods for assembly in diblock copolymer templates

Poly(ethylene oxide)-covered CdSe nanorods were prepared and assembled in diblock copolymer templates by floating the block copolymer templates onto aqueous nanorod solutions. The assembly was enabled by consideration of the surface ligand coverage of the nanorods. Alkane-covered CdSe nanorods prepared by state-of-the-art techniques are not compatible with this assembly process. However, poly(ethylene oxide) (PEO)-functionalized CdSe nanorods were successfully used to assemble the nanorods into the channels and pores of diblock copolymer templates. Other water-dispersible CdSe nanorods, such as those covered with 11-mercaptoundecanoic acid (MUA), did not give the desired assemblies. These results are understood by considering the surface energies of the PEO-covered CdSe nanorods in this interfacial assembly process.     

Recognition-mediated assembly of nanoparticles into micellar structures with diblock copolymers

Polystyrene-based diblock copolymers, featuring diaminotriazine functionality on one of the blocks were used to assemble complementary uracil-functionalized nanoparticles into micellar aggregates. The size of these self-assembled aggregates was controlled by block length, as determined in solution (using dynamic light scattering), and in thin films (using transmission electron microscopy).     

Two-dimensional assembly of giant molecules

This mini review summarizes recent progress on two-dimensional (2D) self-assembly of giant molecules. Two critical factors with significant impact on the formation of nanostructure are highlighted, i.e., the dimensional constraint of 2D geometry, and the conformational constraint imposed the rigid molecular nanoparticles (MNPs). Diverse 2D nanostructures have been fabricated in condensed state and solution by rational molecular design. The collective secondary interactions between functional groups on the periphery of the MNPs and their persistent shape, together with the dimensional limit, change the free energy landscape and lead to unconventional nanostructures. The unique molecular properties of giant molecules endow these 2D structures with promising technological applications.     

Synthesis and self‐assembly of rod‐coil molecules with n‐shaped rod building block

The rod‐coil molecules with n‐shaped rod building block, consisting of an anthracene unit and two biphenyl groups linked together with acetylenyl bonds at the 1,8‐position of anthracene as a rigid rod segment, and the alkyl or alkyloxy chains with various length (i.e., methoxy‐ ( 1 ), octyl‐ ( 2 ), hexadecyl‐ ( 3 )) at the 10‐position of anthracene and poly(ethylene oxide) with the number of repeating units of 7 connected with biphenyl as coil segments were synthesized. The molecular structures were characterized by ¹H NMR and MALDI‐TOF mass spectroscopy. The self‐assembling behavior of new type of molecules 1–3 was investigated by means of DSC, POM, and SAXS at the bulk state. These molecules with a n‐shaped rod building block segment self‐assemble into supramolecular structures through the combination of π–π stacking of rigid rod building blocks and microphase separation of the rod and coil blocks. SAXS studies reveal that molecules 1 and 2 show hexagonal columnar and rectangular columnar structures in the liquid crystalline phase, respectively; meanwhile, molecules 1–3 self‐organize into lamellar structures in the crystalline state. In addition, self‐assembling studies of molecules 1–3 by DLS and TEM indicated that these molecules self‐assemble into elongated nanofibers in aqueous medium. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1415–1422, 2010     

Liquid-crystalline polymers from cationic dendronized polymer-anionic lipid complexes

The use of cationic dendronized polymers as a polyelectrolytic system for templating thermotropic liquid-crystalline phases (LC) via complexation and self-assembly with counter-charged ionic lipids is described. The topology of the LC phases resulting from the self-assembly process, their lattice parameter, and the interpenetration of lipid chains is discussed via birefringency analysis and small-angle X-ray scattering. Depending on the generation of the dendronized polymer and the length of the alkyl chains, amorphous, lamellar, and columnar tetragonal phases are observed. A structural model is proposed which accounts for the systematic variations of alkyl chain length as well as polymer generation. Owing to the reversible nature of the ionic complexation, this process proves high relevance for nanoporous channels, biomimetic, transport, and nanotemplating applications.     

Liquid-crystalline and glass-forming dimers derived from 1-aminopyrene

A homologous series of symmetric dimeric liquid crystals has been synthesized and its phase behaviour investigated. All members of the series were found to form glassy phases above room temperature.     

Liquid crystal molecules with an enyne rigid core

New mesogens are always a source of interest, especially when they possess a non-conventional architecture. In this article are presented the synthesis and polymorphism of a series of compounds possessing a 1,4-diaryl-1-buten-3-yne moiety as the rigid core with an alkoxy chain on each side. Such a core is termed an enyne core. The alkoxy chain is lengthened on each side of the enyne core according to two different fashions: symmetrically and asymmetrically. In this way a rich polymorphism is achieved in some compounds. At lower chain length, the compounds exhibit smectic H and nematic phases where cybotactic groups are observed in X-ray diffraction patterns. As the alkoxy chains extend, smectic C and smectic F phases appear. The non-cylindrical shape of these compounds involves a molecular packing that is preserved throughout the polymorphism. A comparison between symmetric and asymmetric compounds, from X-ray diffraction pattern analysis of their smectic H phases, reveals a parallel molecular stacking. It also discloses the importance of the moiety that is lengthened since different polymorphisms are obtained.     

Liquid-crystalline main chain polyesters derived from p-phenylenediacrylic acid

4,4′-(p-phenylenediacryloyloxy)-dibenzoyl chloride was polycondensed at 220°C with several diols in order to obtain liquid-crystalline polymers. The mesomorphic properties were studied by optical microscopy and differential scanning calorimetry. Gelation due to thermal crosslinking occurred at higher temperatures. With the exception of 2,2′-biphenyldiol derivative, all the polymers derived from aliphatic diols, dihydroxy ethers and aromatic diphenols exhibit mesogenic behaviour.     

Liquid crystal molecules with an enyne rigid core

New mesogens are always a source of interest, especially when they possess a non‐conventional architecture. In this article are presented the synthesis and polymorphism of a series of compounds possessing a 1,4‐diaryl‐1‐buten‐3‐yne moiety as the rigid core with an alkoxy chain on each side. Such a core is termed an enyne core. The alkoxy chain is lengthened on each side of the enyne core according to two different fashions: symmetrically and asymmetrically. In this way a rich polymorphism is achieved in some compounds. At lower chain length, the compounds exhibit smectic H and nematic phases where cybotactic groups are observed in X‐ray diffraction patterns. As the alkoxy chains extend, smectic C and smectic F phases appear. The non‐cylindrical shape of these compounds involves a molecular packing that is preserved throughout the polymorphism. A comparison between symmetric and asymmetric compounds, from X‐ray diffraction pattern analysis of their smectic H phases, reveals a parallel molecular stacking. It also discloses the importance of the moiety that is lengthened since different polymorphisms are obtained.     

Colloidal assembly: the road from particles to colloidal molecules and crystals

Colloidal particles may be considered as building blocks for materials, just like atoms are the bricks of molecules, macromolecules, and crystals. Periodic arrays of colloids (colloidal crystals) have attracted much interest over the last two decades, largely because of their unique photonic properties. The archetype opal structures are based on close-packed arrays of spheres of submicrometer diameter. Interest in structuring materials at this length scale, but with more complex features and ideally by self-assembly processes, has led to much progress in controlling features of both building blocks and assemblies. The necessary ingredients include colloids, colloidal clusters, and colloidal "molecules" which have special shapes and the ability to bind directionally, the control over short-range and long-range interactions, and the capability to place and orientate these bricks. This Review highlights recent experimental and theoretical progress in the assembly of colloids larger than 50 nm.     

Layer-by-layer assembly of single-charged ions with a rigid polyampholyte

A novel method to produce multilayer films has been developed by layer-by-layer assembly of single-charged ions and a rigid polyampholyte containing unbalanced charges in each of its repeat units.